The production of non-oriented electrical steel both at home and abroad has gradually entered into the era of excess capacity, and low-grade oriented silicon steel products have also stepped into the stage of saturation. In order to secure the products a place in the fierce competition in the market, it is of great significance to continue to achieve product quality upgrade, or continue to reduce production cost. The production methods for silicon steel include steelmaking, hot rolling, normalizing, acid pickling, cold rolling and subsequent annealing. Non-oriented silicon steel is often subject to normalizing treatment for the purpose of obtaining a coarse and large grain structure for the hot rolled sheet before cold rolling, so as to achieve a high-strength 0vw texture for the cold-rolled sheet upon annealing. The normalizing of oriented silicon steel products is aimed at adjusting the grain size and texture, realizing hard-phase control, generating free C and N, precipitating ALN and so on.
If the normalizing process is not properly controlled, that is, in the actual production process, if the energy input rate is not effectively controlled, the excess coefficient won't realize the stable control of <1.0, and the actual excess coefficient will be >1.0. As a result, there will be excess oxygen concentrated locally in the furnace, and the reducing atmosphere won't be maintained in the whole non-oxidation heating furnace section. The local excess oxygen will react with Si, Al, Mn, etc., and form on the substrate surface a layer of hardly removable dense oxides constituted of Si, Al, Mn, etc. These oxides adhering to the surface of the substrate will be extremely difficult to be removed in the subsequent shot blasting and acid pickling treatment. After cold rolling, dustlike point and strip-shaped hand feeling-free matters will be found attached locally or entirely across its width on the surface of the rolled hard sheet.
Japan is a world leader in terms of silicon steel production technology level. For example, the Japanese laid-open Patent Publication SHO 48-19048 focused on how to strengthen the acid pickling treatment to remove the dense oxides already produced as much as possible. Domestic published literature, Electrical Steel edited by He Zhongzhi, also discloses how to eliminate the oxides attached on the substrate surface. The specific descriptions are as follows: subject the annealed steel sheet to acid pickling treatment in concentrated hydrochloric acid containing 10% HF or 1˜2% HF +6% HNO3 at 70° C., or subject it to H3PO4 +HF chemical polishing or electrolytic polishing. After complete removal of attached oxides, subject the substrate to subsequent treatment, and the iron loss of the finished silicon steel products will be significantly reduced.
The above literature all propose the strengthening of acid pickling treatment to remove dense oxides on the substrate surface in the steps following normalizing, but they are only follow-up remedial measures. There are usually such problems as complicated process and increased cost in subsequent steps after normalizing. Therefore, efforts are still expected to be made to prevent the formation of dense oxides in the normalizing treatment process.